U.S. patent number 5,661,750 [Application Number 08/473,011] was granted by the patent office on 1997-08-26 for direct sequence spread spectrum system.
This patent grant is currently assigned to CellNet Data Systems, Inc.. Invention is credited to Forrest F. Fulton.
United States Patent |
5,661,750 |
Fulton |
August 26, 1997 |
Direct sequence spread spectrum system
Abstract
A direct-sequence spread spectrum communication system using a
high power transmitter and a short spreading sequence which still
satisfies FCC Rule 15.247 regarding power density. In addition to
the spreading sequence, the carrier signal is modulated with a
phase reversal sequence. Typically, each period of the phase
reversal sequence has a duration equal to the total duration of the
spreading sequence. The phase reversal sequence reduces the maximum
power density of the signal, but is transparent to a receiver.
Inventors: |
Fulton; Forrest F. (Los Altos
Hills, CA) |
Assignee: |
CellNet Data Systems, Inc. (San
Carlos, CA)
|
Family
ID: |
23877818 |
Appl.
No.: |
08/473,011 |
Filed: |
June 6, 1995 |
Current U.S.
Class: |
375/141; 375/146;
375/367; 375/E1.002 |
Current CPC
Class: |
H04B
1/707 (20130101); H04L 27/18 (20130101) |
Current International
Class: |
H04L
27/18 (20060101); H04B 1/707 (20060101); H04K
001/00 () |
Field of
Search: |
;375/200,354,206,208,279,295,316,329,367 ;364/717 ;327/164
;370/105.1,18 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bocure; Tesfaldet
Attorney, Agent or Firm: Fish & Richardson, P.C.
Claims
What is claimed is:
1. A spread spectrum transmitter apparatus, comprising:
a signal source to generate a signal having a first bandwidth;
a first modulation function which modulates said signal with a
pseudo-random pattern to spread said signal across a second
bandwidth broader than said first bandwidth, said pseudo-random
pattern having a first periodicity equal to a first duration;
a second modulation function which modulates a preamble of said
signal with a phase reversal pattern, other than said pseudo-random
pattern or a data pattern, said phase reversal pattern being
transparent to an acquisition controller of a spread spectrum
receiver, said phase reversal pattern selected to reduce the
maximum power density of said signal by spreading the power density
more uniformly through said second bandwidth by reducing spectral
line separation, said phase reversal pattern including a series of
phase reversal periods, each of said periods having a second
duration; and
an antenna to transmit said signal as modulated by said first and
second modulation functions.
2. The apparatus of claim 1 wherein said second duration is equal
to an integer multiple of said first duration.
3. The apparatus of claim 2 wherein said second duration is equal
to said first duration.
4. The apparatus of claim 1 wherein said pseudo-random pattern is a
modulation sequence having an integer number of chips.
5. The apparatus of claim 4 wherein said integer number is less
than 255.
6. The apparatus of claim 5 wherein said integer number is at most
63.
7. The apparatus of claim 1 wherein said second duration is equal
to an integer multiple of a bit time.
8. The apparatus of claim 1 wherein said phase reversal pattern is
a pseudo-random sequence.
9. The apparatus of claim 8 wherein said pseudo-random sequence is
+1, +1, +1, -1, -1, +1, -1.
10. The apparatus of claim 1 wherein said phase reversal pattern is
a sequence containing only odd-harmonics.
11. The apparatus of claim 10 wherein said odd-harmonic sequence is
-1, -1, +1, -1, +1, +1, +1, +1, -1, +1, -1, -1.
12. The apparatus of claim 1 wherein said phase reversal pattern is
a ternary odd-harmonic sequence.
13. The apparatus of claim 12 wherein said ternary odd-harmonic
sequence is -1, -1, +1, -1, 0, +1, +1, +1, -1, +1, 0, -1.
14. The apparatus of claim 1 wherein said apparatus further
comprises a data source to generate a data information pattern, and
a third modulation function to modulate said signal with said data
information pattern.
15. The apparatus of claim 14 wherein said second modulator does
not modulate said signal during the transmission of data
information pattern.
16. The apparatus of claim 14 wherein said second modulator
modulates said signal during the transmission of data information
pattern.
17. The apparatus of claim 14 wherein said phase reversal pattern
is a pseudo-random sequence.
18. The apparatus of claim 17 wherein said pseudorandom sequence is
+1, +1, +1, -1, -1, +1, -1.
19. The apparatus of claim 14 wherein said phase reversal pattern
is a sequence containing only odd-harmonics.
20. The apparatus of claim 19 wherein said odd-harmonic sequence is
-1, -1, +1, -1, +1, +1, +1, +1, -1, +1, -1, -1.
21. The apparatus of claim 14 wherein said phase reversal pattern
is a ternary odd-harmonic sequence.
22. The apparatus of claim 21 wherein said ternary odd-harmonic
sequence is -1, -1, +1, -1, 0, +1, +1, +1, -1, +1, 0, -1.
23. A spread spectrum transmitter apparatus, comprising:
means to generate a transmitted signal having a first bandwidth at
a power of one watt;
a first modulation function which modulates said signal with a
pseudo-random pattern to spread said signal across a second
bandwidth broader than said first bandwidth, said pseudo-random
pattern having a periodicity equal to a first duration and a
modulation sequence with an integer number of chips less than
255;
a second modulation function which modulates a preamble of said
signal with a phase reversal pattern to reduce the maximum power
density of said signal, said phase reversal pattern including a
series of phase reversal periods, each of said periods having a
second duration;
an antenna to transmit said signal as modulated by said first and
second modulation functions; and
wherein no more than +8 dBm of power is concentrated in any three
kilohertz bandwidth of said signal.
24. A spread spectrum transmitter apparatus, comprising:
means to generate a transmitted signal having a first bandwidth at
a power greater than 389 milliwatts;
a first modulation function which modulates said signal with a
pseudo-random pattern to spread said signal across a second
bandwidth broader than said first bandwidth, said pseudo-random
pattern having a periodicity equal to a first duration and a
modulation sequence with an integer number of chips equal to or
less than 63;
a second modulation function which modulates a preamble of said
signal with a phase reversal pattern to reduce the maximum power
density of said signal, said phase reversal pattern including a
series of phase reversal periods, each of said periods having a
second duration;
an antenna to transmit said signal as modulated by said first and
second modulation functions; and
wherein no more than +8 dBm of power is concentrated in any three
kilohertz bandwidth of said signal.
25. A spread spectrum communication system, comprising:
(a) a transmitter to generate a transmitted signal, said
transmitter including
i) a signal source to generate a signal having a first
bandwidth,
ii) a data source to generate a data pattern,
iii) a first modulator which modulates said signal with a
pseudo-random pattern to spread said signal across a second
bandwidth broader than said first bandwidth, said pseudo-random
pattern having a first periodicity equal to a first duration,
iv) a second modulator which modulates a preamble of said signal
with a phase reversal pattern, other than said pseudo-random
pattern or said data pattern, said phase reversal pattern selected
to reduce the maximum power density of said signal by spreading the
power density more uniformly through said second bandwidth by
reducing spectral line separation, said phase reversal pattern
including a series of phase reversal periods, each of said periods
having a second duration,
v) a third modulator for modulating said signal with said data
pattern, and
vi) an antenna to transmit said signal as modulated by said first,
second and third modulation functions; and
b) a receiver including
i) an antenna to receive said transmitted signal and provide a
received signal,
ii) a local signal source to generate a local signal,
iii) a fourth modulator for modulating said local signal with said
pseudo-random pattern,
iv) a fifth modulator for modulating said local signal in response
to said received signal, and
v) means for synchronizing said fourth modulator with said first
modulator, said phase reversal pattern being transparent to said
synchronization means.
26. The system of claim 25 wherein said phase reversal pattern is a
pseudo-random sequence.
27. The system of claim 26 wherein said pseudo-random sequence is
+1, +1, +1, -1, -1, +1, -1.
28. The system of claim 25 wherein said phase reversal pattern is a
sequence containing only odd-harmonics.
29. The system of claim 28 wherein said odd-harmonic sequence is
-1, -1, +1, -1, +1, +1, +1, +1, -1, +1, -1, -1.
30. The system of claim 25 wherein said phase reversal pattern is a
ternary odd-harmonic sequence.
31. The system of claim 30 wherein said ternary odd-harmonic
sequence is -1, -1, +1, -1, 0, +1, +1, +1, -1, +1, 0, -1.
32. The system of claim 25 wherein said receiver further includes a
filter for filtering said local signal.
33. The system of claim 32 wherein said receiver has a bandwidth,
and said second duration is greater than the inverse of said
bandwidth.
34. A method of operating a spread spectrum transmitter,
comprising:
generating a signal having a first bandwidth;
modulating said signal with a pseudo-random pattern to spread said
signal across a second bandwidth broader than said first bandwidth,
said pseudo-random pattern having a first periodicity equal to a
first duration;
modulating a preamble of said signal with a phase reversal pattern,
other than said pseudo-random pattern or a data pattern, said phase
reversal pattern being transparent to an acquisition controller of
a spread spectrum receiver, said phase reversal pattern selected to
reduce the maximum power density of said signal by spreading the
power density more uniformly through said second bandwidth by
reducing spectral line separation, said phase reversal pattern
including a series of phase reversal periods, each of said periods
having a second duration; and
transmitting said signal as modulated with said pseudo random
pattern and said phase reversal pattern.
35. A method of operating a spread spectrum system, comprising:
generating a signal having a first bandwidth at a transmitter;
modulating said signal at said transmitter with a pseudo-random
pattern to spread said signal across a second bandwidth broader
than said first bandwidth;
modulating a preamble of said signal at said transmitter with a
phase reversal pattern, other than said pseudo-random pattern or a
data pattern, said phase reversal pattern selected to reduce the
maximum power density of said signal without increasing the
bandwidth said signal beyond said second bandwidth by reducing
spectral line separation;
transmitting said signal as modulated with said pseudo random
pattern and said phase reversal pattern;
receiving said transmitted signal at a receiver; and
synchronizing said received signal at said receiver, said phase
reversal pattern being transparent in said synchronization
step.
36. A spread spectrum transmitter apparatus, comprising:
a signal source to generate a signal having a first bandwidth;
a first modulation function which modulates said signal with a
pseudo-random pattern to spread said signal across a second
bandwidth broader than said first bandwidth, said pseudo-random
pattern having a first periodicity equal to a first duration;
a second modulation function which modulates a preamble of said
signal with a phase reversal pattern, other than said pseudo-random
pattern or a data pattern, said phase reversal pattern selected to
reduce the maximum power density of said signal by spreading the
power density more uniformly through said second bandwidth by
reducing spectral line separation, said phase reversal pattern
including a series of phase reversal periods, each of said periods
having a second duration; and
an antenna to transmit said signal as modulated by said first and
second modulation functions.
37. A spread spectrum transmitter apparatus, comprising:
a signal source to generate a signal having a first bandwidth;
a first modulation function which modulates said signal with a
pseudo-random pattern to spread said signal across a second
bandwidth broader than said first bandwidth, said pseudo-random
pattern having a first periodicity equal to a first duration;
a second modulation function which modulates a preamble of said
signal with a phase reversal pattern, other than said pseudo-random
pattern or a data pattern, said phase reversal pattern being
transparent to an acquisition controller of a spread spectrum
receiver, said phase reversal pattern selected to reduce the
maximum power density of said signal, said phase reversal pattern
including a series of phase reversal periods, each of said periods
having a second duration; and
an antenna to transmit said signal as modulated by said first and
second modulation functions.
Description
BACKGROUND OF THE INVENTION
The present invention relates to direct-sequence spread spectrum
radio communications systems, and particularly to systems using a
short spreading sequence while still achieving low power
density.
In standard radio communications, information transmission is
accomplished by modulating a carrier signal in response to data. In
spread spectrum communications, the carrier signal is additionally
modulated by a spreading function. The advantages of spread
spectrum communications include security, reduced interference, and
compliance with Federal Communications Commission (FCC) Rules.
Spread spectrum communication systems which comply with FCC Rules
may operate without a license. Spread spectrum communications may
be used for alarm systems, smoke detectors, utility metering
systems, personal and automobile locators, and other applications
involving many transmitters but few receivers.
FCC Rule 15.247 imposes four requirements on the spread spectrum
communication system. First, the transmitter power cannot exceed
one watt. Second, the receiver processing gain must be at least +10
decibels (dB). Third, the spectrum must be spread across at least a
500 kilohertz (kHz) bandwidth. Fourth, and most importantly for the
purposes of the present invention, there is a maximum power density
restriction. No more than +8 decibels above one milliwatt (dBm) of
power may be concentrated within any three kilohertz bandwidth.
A typical spreading function is a pseudo-random pattern composed of
a repeating sequence. The repeating sequence has a set number of
periods or "chips" each of the same duration. During each period a
phase, amplitude, or frequency modulation is applied to the signal.
A pseudo-random sequence is selected so that the pattern appears to
be noise. As used herein, the "length" of a pseudo-random sequence
refers to the number of chips in the sequence, whereas the
"duration" of a pseudo-random sequence refers to the amount of time
for the communication system to pass through the sequence.
A typical transmitter of a pseudo-random signal generates a
carrier, applies the pseudo-random modulation to the carrier, and
then applies data modulation to the pseudo-randomly modulated
carrier. As is well known in the art, one effect of the application
of a pseudo-random sequence to the carrier signal is to spread the
spectrum power over a wide band. A transmitter to be certified
under FCC Rule 15.247 must spread signal power over at least 500
kHz. This may be accomplished by using chips shorter than about two
microseconds, for a biphase form of spreading modulation.
A typical receiver for a pseudo-random signal uses a local
oscillator to heterodyne the received signal to an intermediate
frequency. By modulating the local oscillator signal with the same
pseudo-random sequence, the spectrum spreading is cancelled, and
the received signal fits into a relatively narrow bandwidth
intermediate frequency filter. Such a system can meet the
processing gain required by FCC Rule 15.247 with a pseudo-random
sequence shorter than 63 chips, and possibly as short as 15
chips.
In order for the spectrum spreading to cancel and produce
intelligible data, the pseudo-random sequence generated in the
receiver must be time-synchronized with the sequence of the
received signal. A typical procedure for acquiring synchronization
is to step the starting point of the locally generated sequence
through the range of the sequence while monitoring the amount of
signal power coming through the narrow intermediate frequency
filter. When the locally generated sequence is not synchronized,
the spectrum remains spread, and only a small amount of the
received signal power passes through the intermediate frequency
filter. However, when the locally generated sequence is nearly
synchronized to the correct timing, the spreading is cancelled,
most of the received signal power fits into the narrow band of the
intermediate frequency filter, and the monitored signal power
increases by the processing gain of the system. After achieving a
rough synchronization, the system stops stepping through the
pseudo-random sequence, and makes smaller shifts to refine the
accuracy of the synchronization.
If a spread spectrum communication system has many transmitters
sending to one receiver, the signals from the various remote
transmitters will generally not arrive with the same sequence
timing. Therefore, the synchronization acquisition procedure has to
be repeated at the start of each packet. This is accomplished by
starting each packet with a preamble which does not contain any
data modulation. The duration of the preamble is sufficient for the
receiver to go through the worst-case synchronization search,
adjust the local sequence to the correct timing, and receive the
signal with full processing gain, before the start of any data
modulation. If the preamble is too short and the receiver fails to
synchronize before the data starts, then data errors will occur.
This is a particular problem in communication systems which use
time division multiple access to communicate short packets from
many transmitters to one receiver.
From the point of view of communication efficiency, the preamble is
wasted time. Therefore, it is desirable to use as short a duration
for the preamble as possible while retaining reliable
synchronization acquisition. The length of time required to acquire
synchronization is approximately proportional to the length of the
pseudo-random sequence. This is because, in the worst-case
scenario, the acquisition process must step through the full length
of the sequence before discovering the correct timing. Typically,
the duration of each step is proportional to the length of the
sequence, but this is not required. Thus, a short pseudo-random
sequence can provide a short preamble time.
In addition to the rule regulating bandwidth spread and processing
gain, the FCC imposes a spectral power density rule. FCC Rule
15.247 requires that the signal power must be evenly spread over
the bandwidth so that no more than +8 dBm of power may be
concentrated within any three kilohertz bandwidth. In the prior
art, the maximum power density is approximately inversely
proportional to the length of the pseudo-random sequence.
Two methods of compliance with the spectral power density rule have
been used. First, the communication system may use a long
pseudo-random sequence. For example, a sequence length on the order
of 255 chips will spread the power sufficiently for a one watt
transmitter to meet the +8 dBm rule. However, such a long
pseudo-random sequence produces a long acquisition time and a very
wasteful preamble.
Second, the communication system may use a low power transmitter.
For example, if a 389 milliwatt transmitter is used, the power
density rule will be met if the sequence length is about 63 chips.
However, a low power transmission is more susceptible to
interference. It would be preferable to use a high-power
transmitter so that the receiver could be spaced further away.
Thus, according to the prior art, the power density rule creates a
trade-off between lower power transmitters, with a higher
susceptibility to interference, and longer sequence lengths, with
reduced communication efficiency.
In view of the foregoing, it is an object of the present invention
to provide a communication system having a short preamble and a
high transmitter power, while still meeting the FCC power density
rule.
Additional objects and advantages of the invention will be set
forth in the description which follows, and in part will be obvious
from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out in the claims.
SUMMARY OF THE INVENTION
The present invention is directed to spread spectrum transmitter
having an apparatus to modulate a signal. The apparatus includes a
first modulator which modulates the signal with a pseudo-random
sequence to spread the signal across a wider bandwidth. The
apparatus also includes a second modulator which modulates the
signal with a phase reversal pattern to reduce the maximum power
density of the signal.
BRIEF DESCRIPTION OF THE DRAWING
The accompanying drawings, which are incorporated in and constitute
a part of the specification, schematically illustrate a preferred
embodiment of the invention and, together with the general
description given above, and the detailed description below, serve
to explain the principles of the invention.
FIG. 1 is a schematic illustration of a radio communication
system.
FIG. 2 is a block diagram which schematically illustrates a prior
art spread spectrum transmitter.
FIG. 3 is a block diagram which schematically illustrates a spread
spectrum transmitter according to the present invention.
FIG. 4 is a schematic block diagram of a circuit to carry out the
phase reversal function of the present invention.
FIG. 5A is a block diagram which schematically illustrates a spread
spectrum receiver.
FIG. 5B is an illustration demonstrating the insensitivity of an
envelope detector to phase reversal.
FIG. 6 illustrates a pseudo-random pattern suitable for spreading
the bandwidth of a signal.
FIG. 7 is a graph of the amplitude of the signal as a function of
frequency, from a prior art transmitter using the pseudo-random
sequence shown in FIG. 6 at a chip rate of 1.8144 MHz.
FIG. 8 illustrates a pseudo-random phase reversal pattern according
to the present invention.
FIG. 9 is a graph of the amplitude of the signal as a function of
frequency, from a transmitter according to the present invention
using the phase reversal pattern shown in FIG. 8.
FIG. 10 is a graph of the amplitude of the signal as a function of
frequency, from a prior art transmitter using the pseudo-random
sequence shown in FIG. 6 at a chip rate of 1.2096 MHz.
FIG. 11 illustrates an odd-harmonic phase reversal pattern
according to the present invention.
FIG. 12 is a graph of the amplitude of the signal as a function of
frequency, from a transmitter according to the present invention
using the odd-harmonic phase reversal pattern shown in FIG. 11.
FIG. 13 illustrates a ternary odd-harmonic phase reversal pattern
according to the present invention.
FIG. 14 is a graph of the amplitude of the signal as a function of
frequency, from a transmitter according to the present invention
using the ternary odd-harmonic phase reversal pattern shown in FIG.
13.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in FIG. 1, a communication system 5 contains may have a
plurality of transmitters 20 and a single receiver 50. By
increasing the power of transmitters 20, the transmitters 20 can be
located further away from receiver 50. Communication system 5 might
be a cellular communication system, but the invention may be
applied to any sort of communication system.
In the preferred embodiment, information is transmitted from
transmitters 20 to receiver 50 in short data packets. Each packet
contains a preamble during which no data modulation occurs and a
data transmission period during which the carrier is data
modulated. The receiver 50 uses the preamble to synchronize with
the received signal.
Certain parts of a prior art spread spectrum transmitter 10 are
shown by block diagram in FIG. 2. In transmitter 10, a carrier
generator 12 provides a carrier signal at a specific frequency. In
the preferred embodiment, the specific frequency is suitable for
radio communication. In its simplest form, the carrier signal may
be conceived as a single-frequency sine wave. The carrier signal is
modulated by the spreading modulator 14. Spreading modulator 14
applies a pseudo-random sequence to modulate the carrier signal,
and expands the bandwidth of the one-frequency carrier signal to
occupy at least a 500 kHz bandwidth as required by FCC Rule 15.247.
The spread signal is then passed through a data modulator 16. Data
modulator 16 modulates the signal in response to an information
signal from some data source (not shown). In a preferred
embodiment, the spreading modulation may be biphase modulation.
Data modulator 16 may use the same form of modulation, such as
phase, frequency, or amplitude modulation, and share the same
hardware, as the spreading modulator 14, but this is not required.
Finally, transmitter 10 generates a radio signal which is
transmitted from antenna 18.
As previously described, FCC Rule 15.247 restricts the maximum
power density in any three kilohertz bandwidth to +8 dBm. The most
direct approach to meet the FCC restriction is to use a lower power
transmitter. However, a low power transmitter makes communication
system 5 more susceptible to interference. An alterative approach
is to use a longer spreading function to produce a larger number of
spectral lines. However, if a longer spreading function is used, as
discussed, then the receiver will take a longer time to synchronize
the signals.
The improved transmitter 20 constructed in accordance with the
present invention is shown in block diagram in FIG. 3. Certain
elements of transmitter 20 not relevant to the invention have been
omitted. Such elements may be provided according to practices well
known to those in the art. Transmitter 20 includes a carrier
generator 22, a spreading modulator 24, a data modulator 26, a
phase reversal modulator 30, and a radio transmitting antenna 28.
In accordance with FCC Rule 15.247, the carrier frequency may be
within the band 902 MHz to 928 MHz.
Phase reversal modulator 30 reverses the phase of the signal with a
pattern that increases the number of spectrum lines produced by
transmitter 20 and thereby decreases the power density of the
broadcast signal. However, the phase reversal pattern is
transparent to the search process used by receiver 50 for the
spreading modulation, so that a short pseudo-random sequence may be
used for spreading the bandwidth. This permits a high power
transmitter to have a fast acquisition time, and still meet FCC
Rule 15.247 for power density.
In one preferred embodiment, the phase reversal pattern continues
during the data modulation but is transparent to the amplitude
modulated data. The duration of each period of the phase reversal
pattern may be equal to an integer multiple of the duration of a
bit of data. Preferably, each period of the phase reversal pattern
is the same duration as one bit of data.
In an alternate embodiment, the phase reversal pattern may be
turned off when the data modulation starts, because the data
modulation itself has the effect of increasing the spectral spread.
For this purpose, a switch (not shown) may control phase reversal
modulator 30 to shut it off when data modulation is occurring by
means of data modulator 30. The switch may be triggered by the end
of the preamble.
A block diagram of phase reversal modulator 30 is shown
schematically in FIG. 4. Phase reversal modulator 30 includes a
phase modulator 32, such as a BPSK Modulator produced by
Mini-Circuits, which applies a 180.degree. phase shift to the
signal 34 from data modulator 26 in response to an input line 35 to
create output 36. a reversal pattern generator 38, which may be
constructed according to practices well known in the art, may
generate a transistor-transistor logic (TTL) compatible signal on
input line 35. Output signal 36 is applied to transmitter antenna
28. Phase reversal modulator 30 is shown as a separate component
for clarity of discussion, but those skilled in the art will
recognize that in some implementations phase reversal modulator 30
may share hardware with spreading modulator 24 and data modulator
26.
A receiver 50 constructed in accordance with the present invention
is shown in block diagram in FIG. 5A. The receiver 50 synchronizes
the locally generated pseudo-random sequence with the timing of the
sequence modulation on the received signal. The receiver 50 does
not require an additional function to remove the phase reversal
pattern in order to properly receive and descramble the spread
signal generated by improved transmitter 20. As will be explained,
the receiver 50 does not need to remove the phase reversal pattern
from the received signal to acquire synchronization with the
spreading pattern.
In receiver 50, a local oscillator 52 generates a local sine wave
signal offset from the transmitter frequency by the intermediate
frequency, such as 10.7 MHz for a carrier frequency of 915 MHz.
This local signal is modulated by a spreading modulator 54 with the
same pseudo-random sequence as spreading modulator 24. Mixer 56
modulates the local signal in response to the received signal
picked up by receiving antenna 58. The local signal is then passed
through an intermediate frequency filter 60 which has a fairly
narrow bandwidth, such as about 100 KHz. The output of filter 60 is
detected by envelope detector 62.
As is well known in the art, if the spreading modulator 54 is not
synchronized with the pseudo-random sequence on the incoming
packet, then the local signal remains spread and only a small
portion passes through the intermediate frequency filter 60. On the
other hand, if the two pseudo-random sequences are synchronized,
then the spreading is cancelled, and the full power of the received
signal passes through intermediate filter 60.
An acquisition controller 64 controls spreading modulator 54 to
find synchronization. Acquisition controller 64 steps through the
local pseudo-random sequence. The voltage output of the envelope
detector 62 is fed into the acquisition controller 64, and changes
in the voltage are compared. In particular, the acquisition
controller 64 searches for a peak in voltage from envelope detector
54 which is greater than voltage at unsynchronized conditions by
the processing gain of the receiver.
In the preferred implementation of a spread spectrum receiver, at
each test step, acquisition controller 64 measures the voltage from
envelope detector 62 for the full length of the spreading sequence.
The voltage for the test step is measured and stored, the starting
point of the pseudo-random sequence generated by spreading
modulator 54 is incremented, and the voltage for the new test step
is measured and stored again. If the new voltage is not
significantly higher than the stored voltage value, then the search
continues in another test step. The incrementing process continues
until the voltage increases. Then the acquisition controller
switches to a finer search to peak the voltage and perfect the
synchronization between the locally generated sequence and the
sequence on the incoming packet.
The present invention utilizes the fact that the envelope detector
62 and therefore the acquisition controller 64 are not sensitive to
the phase of the incoming carrier signal. As shown by FIG. 5B, an
envelope detector, such as a standard linear diode detector,
produces the same output voltage 67 from original signal 68 during
period T.sub.1 as during period T.sub.2 when the phase of the input
signal 69 has been shifted by 180.degree.. Because detectors for
both amplitude modulation and frequency modulation incorporate
envelope detection, the phase reversal modulation of the signal
will be transparent for the purpose of both acquisition control and
data reception for these modulators. In general, as realized in the
present invention, an additional phase reversal modulating function
can be imposed by the transmitter during the preamble of the signal
without modifying the receiver. As long as the frequency of the
reversals is low compared to the bandwidth of the intermediate
frequency filter, the acquisition controller will function
correctly. For example, if the intermediate frequency filter
bandwidth is about 100 KHz, then the duration of each phase
reversal period should be greater ten microseconds, and more
preferably greater than twenty microseconds. Typically, the
duration of each reversal period is an integer multiple, such as
one or two, of the duration of the entire spreading sequence.
The ability of the present invention to reduce the power density of
the signal compared to prior art systems will first be discussed
with reference to FIGS. 6-9. FIG. 6 illustrates a pseudo-random
pattern 70 suitable for spreading the bandwidth of a signal. The
pseudo-random pattern is a sixty-three chip sequence, total
duration T.sub.s, of pseudo-random +1 and -1 values. A graph of the
amplitude of the carrier spread by the pseudo-random pattern of
FIG. 6, as a function of frequency, is shown in FIG. 7. The
spectral lines shown in FIG. 7 are each a fraction of the original
unspread amplitude, and are calculated under the assumption that
the chip rate is 1.8144 MHz and that the spreading modulator
creates biphase modulation. The lines occur at the harmonics of
1.8144 MHz/63 or every 28.8 kHz. The total duration T.sub.s of
sequence 70 equals 34.7 microseconds.
As shown in FIG. 7, the largest line in the spectrum has an
amplitude of 0.1269 relative to the amplitude of an unspread
signal. Therefore, the power of the largest line in the spread
spectrum is 20 log.sub.10 [0.1269]=17.9 dB below the total power of
the signal. Assuming that the total power of transmitter 10 is one
watt, or 30 dBm, the power of the largest line in the spread
spectrum would be 30-17.9=12.1 dBm. Since FCC Rule 15.247 restricts
the total power in a three kilohertz band to 8.0 dBm, the largest
line would exceed the FCC restriction by 4.1 dBm. Therefore, in
order to comply with the FCC rule, a transmitter 10 using the
sixty-three chip sequence 70 can only operate at 389 milliwatts or
25.9 dBm.
A sample reversal pattern used by transmitter 20 according to the
present invention is illustrated by FIG. 8. The reversal pattern 80
is a pseudo-random pattern seven periods long. Each period T.sub.s1
is as long as the total duration of the pseudo-random spreading
sequence. At a chip rate of 1.8144 MHz, the duration of each period
is 34.7 microseconds (63 chips/1.8144.times.10.sup.6 chips/sec).
The pseudo-random sequence may be expressed as +1, +1, +1, -1, -1,
+1, -1, although, of course, the starting point in the sequence is
arbitrary.
The effect on the spectrum from applying the phase reversal pattern
80 in addition to the spreading sequence 70 is shown by the graph
of FIG. 9. Applying the additional phase reversal pattern 80 splits
each line into seven, so that the spectral line separation is
reduced from 28.8 kHz to 4.11 kHz.
As shown in FIG. 9, the largest line in the spectrum has an
amplitude of 0.0795 relative to the amplitude of an unspread
signal. Therefore, the power of the largest line in the spread and
phase-reversed spectrum is 20 log.sub.10 [0.0795]=22 dB below the
total power of the signal. Therefore, for a 30 dBm transmitter, the
power of the largest line will be 30 dBm-22 dBm=8 dBm. By
incorporating the phase reversal pattern of the present invention,
transmitter 20 could use a sequence shorter than 255 chips and
operate at one watt, and still comply with FCC Rule 15.247. In
fact, transmitter 20 could use just a sixty-three chip
sequence.
While the pseudo-random phase reversal pattern 80 is preferred at
high chip rates, it is not appropriate at lower chip rates. This is
because the spectral line spacing becomes too close, and two lines
may fall within a single three kilohertz measurement bandwidth.
FIG. 10 shows the amplitude of the carrier spread by the sequence
of FIG. 6, calculated under the assumption that the chip rate is
1.2096 MHz and that the spreading modulator creates biphase
modulation. At a chip rate of 1.2096 MHz and a pseudo-random
sequence length of sixty-three chips, the spreading modulation
produces spectral lines every 19.2 kHz. Applying the additional
phase reversal pattern 80 will split each line into seven, so that
the spacing between spectral lines is 2.74 kHz. Since the
measurement bandwidth is three kilohertz, two lines may be seen at
once, and a one watt transmitter will violate the FCC rule.
In another embodiment of the present invention, the pseudo-random
phase reversal pattern is changed to a phase reversal pattern that
contains only odd-harmonics of the pattern frequency. When an
odd-harmonic phase reversal pattern is applied to a spread signal,
the resulting spectral lines are separated by twice the pattern
frequency. For all patterns that contain only odd harmonics, the
amplitude value at any given time is equal to the negative of the
amplitude value one-half period earlier or later than that time (E.
A. Guillemin, The Mathematics of Circuit Analysis, John Wiley &
Sons, New York, 1949, pp. 454-457).
A sample odd-harmonic phase reversal pattern 90 according to the
present invention is illustrated by FIG. 11. The phase reversal
pattern 90 is twelve periods long. Each period T.sub.s2 is as long
as the total duration T.sub.s of the spreading sequence. Assuming a
chip rate of 1.2096 MHz and a sixty-three chip sequence, each
period is (63 chips/1.2096.times.10.sup.6 chips/sec) 52.08
microseconds. The sequence may be expressed as -1, -1, +1, -1, +1,
+1, +1, +1, -1, +1, -1, -1. Again, the starting point in the
pattern is arbitrary. The pattern frequency is (12 periods/52.08
microseconds) 1.6 kHz.
The effect of applying the odd-harmonic phase reversal pattern 90
in addition to the spreading sequence 70 is shown by the graph of
FIG. 12. The spectral lines are separated by 3.2 kHz (twice the
pattern frequency of 1.6 kHz), which is sufficiently far apart that
only one line falls within the three kilohertz band for the FCC
rule.
As shown in FIG. 12, the largest line in the spectrum has an
amplitude of 0.0910 relative to the amplitude of an unspread
signal. Therefore, the power in largest line in the spread and
phase-reversed spectrum is 20 log.sub.10 [0.0910]=20.8 dB below the
total power of the signal. A 758 milliwatt, or 28.8 dBm
transmitter, may use the sixty-three chip sequence and still comply
with FCC Rule 15.247. Although this is not as large an improvement
as the pseudo-random reversal pattern at the higher data rate,
there is a 2.9 dB improvement over the system which does not use
the phase reversal pattern.
Another embodiment for the low chip rate case is to use a ternary
odd-harmonic sequence for the phase reversal pattern. The ternary
pattern uses values of +1, -1, and 0. The zero value indicates that
the transmitter is turned off for that period of the pattern.
A sample ternary odd-harmonic phase reversal pattern 100 according
to the present invention is illustrated by FIG. 13. The ternary
reversal pattern 100 is twelve periods long. Again, each period
T.sub.s3 is as long as the total duration T.sub.s of the spreading
sequence. The sequence may be expressed as -1, -1, +1, -1, 0, +1,
+1, +1, -1, +1, 0, -1. Of course, the starting point is
arbitrary.
The effect of applying the phase reversal pattern 100 in addition
to the spreading sequence 70 to a 1.210 MHz chip rate is shown by
the graph of FIG. 14. The spectral lines are separated by 3.2
kHz.
FIG. 14 shows that the largest line in the spectrum has an
amplitude of 0.0719 relative to the amplitude of an unspread
signal. Therefore, the power in largest line in the spread and
phase-reversed spectrum is 22.86 dB below the total power of the
signal. Therefore, a transmitter 20 using ternary phase reversal
pattern 100 may operate at one watt or 30 dBm, use the sixty-three
chip sequence, and still comply with FCC Rule 15.247 with a 0.86
dBm margin. Alternately, transmitter 20 could use a spreading
sequence shorter than sixty-three chips, and comply with the FCC
Rule with a reduced margin. However, the off period in the ternary
phase reversal pattern may not be compatible with other system
considerations, and thus is not applicable to all system
designs.
The invention has been described in terms of biphase modulation by
a spreading modulator, but the invention is not so limited.
According to the present invention, the modulation applied to the
carrier by the spreading modulator, phase reversal modulator, and
data modulator, could be amplitude modulation, frequency
modulation, phase modulation, or a combination thereof. Also, the
phase reversal modulation need not actually be separated from the
spreading modulation in certain implementations.
It is to be understood that spreading modulator 24, data modulator
26, and phase reversal modulator 28 may be physically separated, or
they may be combined and share electronic components. Also, carrier
generator 22 and the modulation functions may be integrated in a
single device. For example, by frequency modulating a carrier
generator, both of those functions are carried out simultaneously.
The modulation functions could be carried out by software or
hardware, and in a digital or analog format.
Furthermore, the order of the elements may be changed. For example,
the data modulation could be applied to the signal before the
signal spreading, or the phase reversal pattern could be applied to
the signal before the data modulation. In addition, the modulation
functions could be applied indirectly by modulating signals from
other functions rather than directly modulating the signal from
carrier generator 22. For example, phase reversal modulator 28
might modulate a data signal or a spreading signal. Any such
reordering or combination of elements is within the scope of the
invention.
The present invention has been described in terms of a preferred
embodiment. The invention, however, is not limited to the
embodiment depicted and described. Rather the scope of the
invention is defined by the appended claims.
* * * * *